"Guy King" wrote in message
...
The message
from The Central Authority contains
these words:

ISTR old fashioned kettles used to go quiet just before starting to
boil vigorously. Presumably once the air has been boiled out of the
region near the element it goes quiet. It's likely the new order
kettles with large surface area heating surfaces just heat the water
too uniformly and gradually prolonging the agony.

Doubtless it's due to Air/O2 bubbles forming and collapsing.

Not quite. The noise before it comes to the boil is steam bubbles
forming and collapsing - the water around the element exceeds the
boiling point but the bulk of the water in the kettle doesn't so the
bubble collapses immediately. The implosion is quite violent, it's the
same idea as an ultrasonic cleaner.

As the bulk of the water nears boiling point it's much closer to the
temperature of the steam in the bubbles, so heat transfer across the
bubble's surface is a lot slower. The bubbles either collapse more
gently, or they fail to collapse at all and break the surface. Either is
a lot quieter than the decrepitation of hot bubbles in cool water.

Of course, more heat is lost when the bubbles break the surface, the
escaping steam carries away heat very efficiently, and this is why water
doesn't exceed its boiling point at a given pressure - the faster you
put heat in the faster the steam carries the heat away. Until of course
it's all gone. Then you need a new element.

--
Skipweasel
Pay no attention to that man behind the curtain.

Nice explanation! Must be the same reason why a milk frother is so noisy
too!

Steve